1. Field of the Invention
The present invention relates to a multilayer printed wiring board for use with electronic devices, components, etc., and more particularly to a multilayer printed wiring board which has a high density and excellent heat dissipating properties.
2. Description of the Prior Art
In parallel with an increasing tendency toward the miniaturization of devices and advancement in the electronic industry, there has been an ever increasing trend aiming at the production of printed wiring boards which permit high-density mounting. Also, there has been a tendency toward forming both a power supply circit and signal circuits on a single printed wiring board (PWB) and further there has been a demand for high-density mounting and improved heat dissipating properties.
In the past, as a PWB capable of ensuring high-density mounting, a multilayer printed wiring board has been used widely in which a plurality of double-sided copper-clad glass epoxy resin boards, each formed with a printed wiring pattern on its laminated copper foil by the subtractive process, are laminated and the necessary conductive paths are provided by plated through-holes. However, the resin boards are made of resins which are inferior in heat conductivity and thick resulting in deterioration of the heat dissipating properties.
Thus, while ceramic multilayer wiring boards utilizing thick film printing have been used widely, they have the disadvantages that they are more expensive, tend to be damaged by mechanical stresses and have limitations in size.
Further while metal cored substrates utilizing good heat dissipating properties of metals have been used in the fields requiring heat dissipating properties, they must have multilayer structures in order to attain the desired high-density mounting. One method (I) to produce a multilayer structure is the use of plated through-holes. More specifically, as shown in FIGS. 2a to 2c, the method comprises laminating a copper foil 23 through an insulation laminate 22 to each side of an aluminum substrate 20 formed with through-holes 21 preliminarily (FIGS. 2a and 2b), forming through-holes 24 in the portions of the copper foils 23 corresponding to the through-holes 21 (FIG. 2c), and forming a plated copper layer on the insides of the through-holes by an electrodeless plating process or an electroplating process.
Another method (II) comprises, as shown in FIG. 3, bonding a metal foil, e.g., copper foil, to one side of a metal substrate 31 through an insulation laminate 32 and removing the unwanted portions of the copper foil by the subtractive process to form a first layer printed wiring pattern 33 of the desired design, and forming interlaminar conductors (hereinafter referred to as through studs), interlaminar insulation laminates and a second layer printed wiring pattern by the use of an insulating material and a conductive paste.
More specifically, as shown in FIG. 3, a through stud 35 made of a cured conductive paste is formed at each of the desired positions on the first layer printed wiring pattern 33 on the insulation laminate 32, and a second layer printed wiring pattern 36 is printed with a conductive paste and cured through interlaminar insulation laminates 34-1 and 34-2.
Then, in order to meet the need for a multilayer printed wiring board which is excellent in heat dissipating properties and low in cost, it is desirable to use a substrate combining the heat conductivity of a metal and the economy and electric insulating properties of a resin. However, of the previously mentioned methods of producing multilayer printed wiring boards of the type employing a metal cored substrate, the previously mentioned method (I) employing the plated through-holes is not suitable for the production of a high-density multilayer printed wiring board which attaches importance to the heat dissipating properties, since the essential aim of using the substrate for its excellent heat dissipating properties is ruined by the presence of the insulation laminates on its sides.
On the other hand, the method (II) employing the conductive paste layers and the insulation laminates is disadvantageous in that the conductivity of the cured conductive paste is low and there is a limitation to the allowable current.
To improve on this point, a method has been proposed in which a metal plating, e.g., copper plating (37 in FIG. 3), is additionally applied onto the cured conductive paste on the top layer printed wiring pattern. However, this method is also disadvantageous in that while the conductivity is increased greatly, a temperature rise due to the flow of a large current is so large that there is a limitation to the allowable current carrying capacity. Such temperature rise is a problem which is mainly concerned with the through studs.